1,139 research outputs found
Spherical magnetic nanoparticles: magnetic structure and interparticle interaction
The interaction between spherical magnetic nanoparticles is investigated from
micromagnetic simulations and ananlysed in terms of the leading dipolar
interaction energy between magnetic dipoles. We focus mainly on the case where
the particles present a vortex structure. In a first step the local magnetic
structure in the isolated particle is revisited. For particles bearing a
uniaxial magnetocrystaline anisotropy, it is shown that the vortex core
orientation relative to the easy axis depends on both the particle size and the
anisotropy constant. When the particles magnetization present a vortex
structure, it is shown that the polarization of the particles by the dipolar
field of the other one must be taken into account in the interaction. An
analytic form is deduced for the interaction which involves the vortex core
magnetization and the magnetic susceptibility which are obtained from the
magnetic properties of the isolated particle.Comment: 20 pages, 10 figures Published in Journal of Applied Physics. To be
found at: http://link.aip.org/link/?jap/105/07391
The synchrotron peak shift during high-energy flares of blazars
A prediction for the energy shift of the synchrotron spectrum of
flat-spectrum radio quasars (FSRQs) during high-energy flares is presented. If
the -ray emission of FSRQs is produced by Comptonization of external
radiation, then the peak of the synchrotron spectrum is predicted to move to
lower energies in the flare state. This is opposite to the well-known broadband
spectral behavior of high-frequency peaked BL-Lac objects where the external
radiation field is believed to be weak and synchrotron-self Compton scattering
might be the dominant -ray radiation mechanism. The synchrotron peak
shift, if observed in FSRQs, can thus be used as a diagnostic to determine the
dominant radiation mechanism in these objects. I suggest a few FSRQs as
promising candidates to test the prediction of the external-Comptonization
model.Comment: 9 pages, including 2 figures; uses epsf.sty, rotate.sty; accepted for
ApJ Letters; minor revision
Field-induced structure transformation in electrorheological solids
We have computed the local electric field in a body-centered tetragonal (BCT)
lattice of point dipoles via the Ewald-Kornfeld formulation, in an attempt to
examine the effects of a structure transformation on the local field strength.
For the ground state of an electrorheological solid of hard spheres, we
identified a novel structure transformation from the BCT to the face-centered
cubic (FCC) lattices by changing the uniaxial lattice constant c under the hard
sphere constraint. In contrast to the previous results, the local field
exhibits a non-monotonic transition from BCT to FCC. As c increases from the
BCT ground state, the local field initially decreases rapidly towards the
isotropic value at the body-centered cubic lattice, decreases further, reaching
a minimum value and increases, passing through the isotropic value again at an
intermediate lattice, reaches a maximum value and finally decreases to the FCC
value. An experimental realization of the structure transformation is
suggested. Moreover, the change in the local field can lead to a generalized
Clausius-Mossotti equation for the BCT lattices.Comment: Submitted to Phys. Rev.
Effects of geometric anisotropy on local field distribution: Ewald-Kornfeld formulation
We have applied the Ewald-Kornfeld formulation to a tetragonal lattice of
point dipoles, in an attempt to examine the effects of geometric anisotropy on
the local field distribution. The various problems encountered in the
computation of the conditionally convergent summation of the near field are
addressed and the methods of overcoming them are discussed. The results show
that the geometric anisotropy has a significant impact on the local field
distribution. The change in the local field can lead to a generalized
Clausius-Mossotti equation for the anisotropic case.Comment: Accepted for publications, Journal of Physics: Condensed Matte
Polarizable molecular interactions in condensed phase and their equivalent nonpolarizable models
Earlier, using phenomenological approach, we showed that in some cases
polarizable models of condensed phase systems can be reduced to nonpolarizable
equivalent models with scaled charges. Examples of such systems include ionic
liquids, TIPnP-type models of water, protein force fields, and others, where
interactions and dynamics of inherently polarizable species can be accurately
described by nonpolarizable models. To describe electrostatic interactions, the
effective charges of simple ionic liquids are obtained by scaling the actual
charges of ions by a factor of 1/sqrt(eps_el), which is due to electronic
polarization screening effect; the scaling factor of neutral species is more
complicated. Here, using several theoretical models, we examine how exactly the
scaling factors appear in theory, and how, and under what conditions,
polarizable Hamiltonians are reduced to nonpolarizable ones. These models allow
one to trace the origin of the scaling factors, determine their values, and
obtain important insights on the nature of polarizable interactions in
condensed matter systems.Comment: 43 pages, 3 figure
Neutrino emission in the hadronic Synchrotron Mirror Model: the "orphan" TeV flare from 1ES 1959+650
A challenge to standard leptonic SSC models are so-called orphan TeV flares,
i.e. enhanced very high energy (VHE) gamma-ray emission without any
contemporaneous X-ray flaring activity, that have recently been observed in
TeV-blazars (e.g., 1ES 1959+650). In order to explain the orphan TeV flare of
1ES 1959+650 observed in June 2002, the co-called hadronic synchrotron mirror
model has been developed. Here, relativistic protons are proposed to exist in
the jet, and interact with reflected electron-synchrotron radiation of the
precursor SSC flare. If the reflector is located in the cloud region, time
shifts of several days are possible between the precursor and the orphan flare.
The external photons, blueshifted in the comoving jet frame, are able to excite
the \Delta(1232)-resonance when interacting with protons of Lorentz factors
\gamma'_p~10^3-10^4. The decay products of this resonance include charged pions
which, upon decay, give rise to neutrino production during the orphan flare. In
this paper we calculate the expected neutrino emission for the June 4, 2002,
orphan TeV flare of 1ES 1959+650. We compare our results with the recent
observations of AMANDA-II of a neutrino event in spatial and temporal
coincidence with the orphan flare of this blazar. We find that the expected
neutrino signal from the hadronic synchrotron mirror model is insufficient to
explain the observed neutrino event from the direction of 1ES 1959+650.Comment: 15 pages, 4 figures, accepted by Ap
Spectral Energy Distributions of Gamma Ray Bursts Energized by External Shocks
Sari, Piran, and Narayan have derived analytic formulas to model the spectra
from gamma-ray burst blast waves that are energized by sweeping up material
from the surrounding medium. We extend these expressions to apply to general
radiative regimes and to include the effects of synchrotron self-absorption.
Electron energy losses due to the synchrotron self-Compton process are also
treated in a very approximate way. The calculated spectra are compared with
detailed numerical simulation results. We find that the spectral and temporal
breaks from the detailed numerical simulation are much smoother than the
analytic formulas imply, and that the discrepancies between the analytic and
numerical results are greatest near the breaks and endpoints of the synchrotron
spectra. The expressions are most accurate (within a factor of ~ 3) in the
optical/X-ray regime during the afterglow phase, and are more accurate when
epsilon_e, the fraction of swept-up particle energy that is transferred to the
electrons, is <~ 0.1. The analytic results provide at best order-of-magnitude
accuracy in the self-absorbed radio/infrared regime, and give poor fits to the
self-Compton spectra due to complications from Klein-Nishina effects and
photon-photon opacity.Comment: 16 pages, 7 figures, ApJ, in press, 537, July 1, 2000. Minor changes
in response to referee report, corrected figure
Lamination And Microstructuring Technology for a Bio-Cell Multiwell array
Microtechnology becomes a versatile tool for biological and biomedical
applications. Microwells have been established long but remained
non-intelligent up to now. Merging new fabrication techniques and handling
concepts with microelectronics enables to realize intelligent microwells
suitable for future improved cancer treatment. The described technology depicts
the basis for the fabrication of a elecronically enhanced microwell. Thin
aluminium sheets are structured by laser micro machining and laminated
successively to obtain registration tolerances of the respective layers of
5..10\^Am. The microwells lasermachined into the laminate are with
50..80\^Am diameter, allowing to hold individual cells within the well.
The individual process steps are described and results on the microstructuring
are given.Comment: Submitted on behalf of EDA Publishing Association
(http://irevues.inist.fr/EDA-Publishing
TAP - Thermal aquifer Potential: A quantitative method to assess the spatial potential for the thermal use of groundwater
This paper proposes a method to assess the potential for thermal use of groundwater and its integration in spatial energy planning. The procedure can be adapted to local regulatory and operational limits, thus estimating legally and technically achievable flow rates and subsequently, the thermal power that can be exchanged with the aquifer through a well doublet.
The constraints applied to flow rates are a drawdown threshold in the extraction well, a limit for the groundwater rise in the injection well and a threshold to avoid the hydraulic breakthrough between the two wells. For the spatial assessment, the hydraulic influence on neighbouring well doublets is simulated with the maximum flow rates before the hydraulic breakthrough occurs. The Thermal Aquifer Potential (TAP) method combines mathematical relations derived through non-linear regression analysis using results from numerical parameter studies. A demonstration of the TAP method is provided with the potential assessment in Munich, Germany. The results are compared with monitoring data from existing open-loop systems, which prove that conservative peak extraction estimates are achieved
Gamma Ray Burst Neutrinos Probing Quantum Gravity
Very high energy, short wavelength, neutrinos may interact with the
space-time foam predicted by theories of quantum gravity. They would propagate
like light through a crystal lattice and be delayed, with the delay depending
on the energy. This will appear to the observer as a violation of Lorenz
invariance. Back of the envelope calculations imply that observations of
neutrinos produced by gamma ray bursts may reach Planck-scale sensitivity. We
revisit the problem considering two essential complications: the imprecise
timing of the neutrinos associated with their poorly understood production
mechanism in the source and the indirect nature of their energy measurement
made by high energy neutrino telescopes.Comment: 14 pages, 2 figure
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